It is well known in the art that the best and perhaps the only practical way of measuring flatness of strip as it is being rolled by a strip rolling mill, with tension applied to incoming and outgoing strip, is to measure the tension distribution across the width of the strip, as it leaves the rolling mill and travels to a coiler or take-up reel, or some other downstream process.
In general, a strip which has uniform tension distribution would lie flat on a horizontal table if it was subsequently unwound from the coiler and set down with the tension removed. Strip having non-uniform tension distribution would, in general, not lie flat, but would be seen to have wavy or buckled portions, corresponding to the zones of the strip which had been rolled with the lowest tension.
An early shape or flatness measuring device is disclosed in Pearson, U.K. 1,160,112 and corresponding U.S. Pat. No. 3,499,306. The Pearson device of FIG. 1 was not a commercial success, but the Pearson device shown in FIG. 7 of the Pearson '306 patent was. This device and the one shown on FIG. 3 of Pearson '306 operated by sensing the tension distribution in the material, by passing it over a measuring roller.
The measuring roller of Pearson '306 consists of a central, stationary ("dead") shaft, and a series of bearings mounted concentrically on the shaft. These bearings are placed side by side across the full width of the strip material. Transducers are provided at each bearing location to measure the radial force on the bearing, this being a measure of the tension in that portion of the material passing over that bearing. The Pearson device in FIG. 7 utilized fluid film bearings, and pressure transducers were used to measure fluid pressure, which is a measure of radial force. The device in FIG. 3 utilized roller bearings, with a flexible portion on each bearing inner race within the load zone of the bearing. A displacement transducer was used to measure the deflection of this flexible portion, this deflection being a measure of radial force.
Another stationary shaft shapemeter is disclosed in Muhlberg (U.S. Pat. No. 3,557,614), which is similar in concept to the FIG. 3 embodiment of Pearson '306 (but with additional features). The essential features in Muhlberg are a series of bearings mounted upon a common shaft, with suitable covering for the bearings and with a force sensing transducer mounted underneath some or all of these bearings, to measure the radial force developed on these bearings as a result of the strip wrapping around the roll under tension.
Yet another stationary shaft shapemeter is disclosed in Flinth (U.S. Pat. No. 3,413,846). Flinth used a shapemeter roll as a billy roll (which is normally understood in the art to be a roll located between a mill and a coiler, and is used to maintain a constant pass line level through the mill, while the coil diameter is building up (coiling), or reducing (uncoiling). The billy roll consisted of a central, stationary shaft, an outer casing rotatably mounted on the shaft, and a number of pressure sensing means arranged to be influenced by the pressure between the outer casing and the shaft.
The way in which all commercially available shapemeters work is by providing a roll around which the strip passes on its way from the rolling mill to the subsequent process. The strip wraps around the roll usually by an angle in the range 5.degree. to 90.degree.. In some applications this angle can be fixed. In others, for example when the roll is used as the only deflector roll (sometimes called a billy roll) between a rolling mill and a coiler, the wrap angle varies as the coil builds up in diameter as rolling progresses and more strip is added to the coil. However, in all cases a radial force develops on the roll as a result of the strip under tension wrapping around it, and shapemeters work by measuring the distribution of this force across the face of the roll, this being a measure of the distribution of tension across the width of the strip.
In these conventional shapemeters, the distribution of force is measured by a row of transducers mounted within this roll, usually spaced at fixed intervals in the range 20-60 mm across the face of the roll. Because the tension at the strip edges is very critical--since excessive tension at the edges can lead to strip breakage, particularly if the strip edges are cracked or otherwise defective--some shapemeters use smaller intervals or pitches in areas of the roll close to the strip edges than in areas close to the middle of the strip. The portion of the roll corresponding to an individual transducer is known as a measuring zone, and each transducer measures the radial force produced by the portion of strip passing around that zone of the roll.
In principle there are two types of shapemeters covered by the above description. The first type utilizes a single roll mounted in bearings. Transducers are mounted within the roll, which is machined to provide cavities in which the transducers can be fitted. Each measuring zone, and hence each transducer, is covered with a thin ring of steel, which itself may be covered in an elastomeric material. The entire roll consists of a body which is sufficiently long to cover the maximum width of strip to be rolled, and an integral neck at each end of the body. Each neck is bearing mounted within fixed housings. The transducers all rotate with the roll, and therefore, they are only subjected to load for a small portion of each revolution of the roll. If the wrap angle of the strip is 30 degrees, for example, the transducers are loaded for only 30 degrees for each revolution, and are unloaded for the remaining 330 degrees.
In order to obtain electrical signals of load from the transducers (which are rotating), it is necessary to provide slip rings or other devices, such as multi-channel inductive pick-ups or FM radio links, to transfer these signals to a computer or other display device that is positioned at a fixed location. Since there are multiple transducers, their signals are typically sampled and combined into one overall load-relative signal, thereby requiring multiple analog or digital comparators to choose the presently active signal (i.e., the signal of the greatest magnitude).
The second shapemeter type utilizes a fixed (non-rotating) shaft which spans across the width of the strip, and is supported in stationary support blocks. A separate bearing is mounted upon this shaft at each measuring zone, and on the outside of this bearing a plain or urethane covered steel ring is mounted, covering the full width of the zone. On the inside of each bearing a fixed transducer is mounted within the shaft, this transducer measuring the radial force on the bearing. The output signal from each transducer can be directly wired to a stationary external computer or other display device, usually through an axial hole passing through the shaft, provided for this purpose. The transducers are loaded for the full 360.degree. rotation of the roll.
Each of the above types of shapemeters has its advantages. The first type has the advantage that the shaft diameter is effectively the full diameter of the roll, and therefore this has greater rigidity and lower shaft stress and deflection than the second type. The second type has the advantage that no slip rings are required, and that the output signal is steady, and does not need to be sampled. The greater shaft deflection may not be significant when tensions are not too high, wrap angles are not too large, or roll face length is not too long. The deflection can also be reduced by using bearings with a very small section height, such as air film or oil film bearings, or by increasing the roll diameter.
One of the problems associated with any type of shapemeter is the range of force that each transducer may be subject to. Whereas, the total strip tension may typically vary over a range of 20:1, and the total radial force on the shapemeter roll would vary by the same amount, the radial force on each individual zone may easily vary by a factor of four (4) greater than this range, i.e. by as much as 80:1.
This is explained, for example, by considering a typical measuring zone close to the edge of the strip, firstly when rolling at maximum total tension, but producing strip in which the tension on the edges is twice as high as the average tension (and tension at the middle is correspondingly much lower than the average tension). Note that tension here would be expressed as lb/in of strip width (or Kg/mm of strip width) and average tension equals total tension (lb) divided by strip width (in.) (or total tension (Kg) divided by strip width (mm)). In this case the maximum tension in the measuring zone is equal to twice the maximum average tension (i.e., twice the maximum total tension divided by the strip width).
If we now consider, secondly, the same measuring zone, but when producing strip in which the tension at the edges is half as high as the average tension (and tension at the middle is correspondingly higher than the average tension). In this case the minimum tension in the measuring zone is equal to half the minimum average tension (i.e., half the minimum total tension divided by the strip width).
Thus, the range of tension measured in this zone is from two (2).times.maximum to one-half (1/2).times.minimum average tension (i.e., it is four (4).times.the ratio of maximum total tension to minimum total tension) and thus, if that maximum-to-minimum total tension ratio is 20:1, the range of tension measured in this typical measuring zone would be 80:1.
The difficulty of making measurements of signals having this wide range is that electrical noise can be significant, and may be as high as the minimum signal level. To some extent this noise can be filtered out, at the penalty of reducing the speed of response of the measuring system. It should be noted that the low end of the measuring range is usually the most critical, as it is here that the best performance is desired. This is true because the finished product of the rolling mill is where the strip has been rolled to the lightest gauge, and it is the flatness of the finished product which is of most concern to the rolling mill operator. The flatness produced on earlier passes through the mill can always be corrected by subsequent passes as the material becomes thinner.
One significant problem in conventional shapemeters is that an even greater range of signal levels are normally expected to occur when a shapemeter of the second type (i.e., stationary shaft type) is used in a variable wrap angle situation. If a load cell is used in each measuring zone to measure the radial component of the strip tension force as the strip wraps around the shapemeter roll, then, for a given strip tension force (in that zone) the radial force will vary with wrap angle. In general, for a wrap angle .theta., the radial force=2.times.sin (.theta./2)
For a typical rolling mill, .theta. may vary in the range from 20.degree.-60.degree. or even more. This angle depends upon the diameter of the mandrel upon which the coil is wound, the maximum diameter of the coil, and the vertical and horizontal spacing between shapemeter roll and the coiler mandrel. Thus the radial force for a given tension may have a range as much as 3 to 1. For the above example where the range of signal level would be 80:1 for a fixed angle of wrap, this range of radial force would increase to 240:1, in the case of variable wrap angle.